She ended up with $80k in her pocket and I was able to negotiate a $20k assignment fee.

You will be able to finance multiple properties at once and minimize closing fees from doing multiple loans. 

What is my next step (what contracts etc do I need) without involving realtors so I don't have to pay extra fees to this and that person?

That being said $500 per month in economic vacancy quickly adds up ($6,000 per year).

(my mortgage will be around 8%)The unit has a min. 3 months rental as per HOA, which is still with the developers on the board. 

The 3% or 4% fee sounds good, but if you are paying them off quickly, the actual interest rate you are paying is over 10%.  

That’s an extra couple grand per month right there. 

But it was near a neighboring city (30-45 minute commute) with a lot of potential value. 

You will gain attention if you put your listing on the MLS as @Celine Crestin mentioned, but you will also likely incur significant lease up fees, potentially on both sides (listing and showing).  

So, the first question they usually ask a PMC is about fees - instead of asking about services and HOW those services are executed.EXAMPLE: PMC states they will handle tenant screening – what does that specifically mean?